SheldonD said:
This is normal practice but in my experience, turning on a piece of equipment often produces a spike that sometimes induces oscillation. Also, with open inputs there is noise etc. An amp that is stable and well laid out will not oscillate. An amp with an oscillation without an exciting force will sometimes just sit there and apparently betray its true nature, hence there needs to be noise.
Motorboating can often be detected by AC connecting the supply rail to an oscilloscope. If the trace will keep disappearing off the screen and coming back. I suppose this test could be done by measuring AC on the supply rail with a dmm.